Deep EM Exploration using High Powered Transmitters and SQUID Sensor Receivers
Historically, geophysical methods, particularly EM, were used to discover mineral deposits beneath the surface of the earth. Prospectors could discover deposits in outcrops, but geophysics was needed to find deposits below the surface. It is estimated that approximately 90% of all VMS metal reserves are located at depths of 200 to 600 metres (Heinneman and Morris, 2002). However, it is known that more mineral deposits are at greater than 1000 m depth. Many of these deep deposits have not been explored adequately due to the challenges of creating enough EM energy to penetrate a kilometre or more deep, and to record the very weak responses from conductive minerals at that depth.
To overcome this limitation, very high-powered transmitters are used to penetrate the EM signal through conductive cover rocks to targets at greater deep. Once the desired transmitters can produce a high enough current to induce EMF into these deep targets, it is necessary to have a sensitive enough EM sensor, such as a SQUID, that will be able to detect weak signals above the ambient noise to increase the chances of detecting EM signals induced in deep conductive mineral deposits.
Low-frequency EM is employed because lower frequencies penetrate deeper. Transient EM measurement in the off-time is favoured because there is no primary field to mask the secondary response. Late-time channels are preferred because the latest response after shutoff is due to the highest conductivity of the deep targets. The signal from the less conductive cover and formational conductors will be observed in the early to mid-time of the secondary response.
The deep conductor at the Lalor deposit is a good example of detecting a deep-seated ore body with a high-power transmitter and a high-precision sensor using low-frequency, late-time transient EM.